It is known to use a tracing milling machine for machining a metallic product having a complicated shape, such as a male metal mold and a female metal mold. The tracing milling machine is always provided with a tracer head to which a tracer with a stylus is attached. The tracer performs a tracing movement over a surface of a model, and in response to the tracing movement of the tracer, a servo movement of a cutting tool for carrying out the machining of the metal product from a workpiece is controlled. The cutting tool attached to a spindle of the tracing milling machine has a predetermined tool radius and a predetermined tool length, although actual values of the tool radius and length may vary from the predetermined values, respectively, within a production tolerance of the cutting tool. When the machining of the metal product from a workpiece is completed, the metal product is shaped in the same or similar shape as that of the traced model.
It is well understood that the machining of a metal mold having a complicated male or female shape usually takes long machining time and, also, that the amount of metal chips cut from a workpiece during the machining process is very great. Accordingly, during the machining process of a metal mould from a workpiece thereof, it is always necessary to exchange a used cutting tool attached to the spindle head for a fresh cutting tool. Further, when a cutting tool attached to the spindle head is broken, such broken cutting tool must be exchanged for a fresh cutting tool. However, no conventional tracing milling machine is provided with a so-called automatic tool changer which enables an automatic tool change during the machining process of a metal product from a workpiece. This is because the employment of a known automatic tool changer by a conventional fully automated machining center for the conventional tracing milling machine causes one non-neglectable inconvenience from the point of view of achieving accurate machining of a metal product which must have precisely the same shape as that of a tracing model. In this regard, in the conventional tracing milling machine, control of the horizontal movement of a work table, on which the workpiece and the model are fixedly mounted, in the X-axis and Y-axis directions as well as control of the associated vertical movements of a tracer head and the spindle head to which a cutting tool is attached in the Z-axis direction are performed in response to the tracing movement of a tracer over the surface of the model, whereby the cutting tool applies a cutting motion to the workpiece on the work table so that the workpiece is shaped into the same shape as that of the model. Therefore, it is inevitably necessary that a predetermined positional relationship between the cutting tool and the tracer be always maintained constant over the entire course of the machining process of a workpiece. Otherwise, no accurate machining of a metal product having the same shape and dimensions as those of the model can be achieved by the tracing milling machine. For this reason, at the initial stage of the machining process of the tracing milling machine, the model and the workpiece are firstly fixed to the work table of the tracing milling machine in such a manner that a predetermined distance is established between the centers of both the model and workpiece. Subsequently, the tracer head and the spindle head are adjusted to that the distance between the axes of the tracer head and the spindle head corresponds to the predetermined distance between the centers of the model and workpiece. Thereafter, the tracer and a cutting tool are attached to the tracer head and the spindle head, respectively, so that the aforementioned predetermined positional relationship between the tracer and the cutting tool is established. After the establishment of the predetermined positional relationship, an actual cutting process of the tracing milling machine is commenced. However, during the cutting process, if the tool changing operation is performed so that the used cutting tool attached to the spindle head is replaced with a fresh cutting tool, the predetermined positional relationship between the tracer and the used cutting tool is often broken, since the tool data of the fresh cutting tool, i.e. the radius and the length of the tool, are usually different from the data of the used cutting tool.
On the other hand, in the machining process of the tracing milling machine, if an automatic tool change could be effected during the process of machining a workpiece, the entire process of the tracing machining of the workpiece, ranging from an initial rough cutting process to a final finish cutting process, could be automatically and successively carried out at a high production efficiency. Accordingly, it is strongly desired that tracing milling machines be provided with appropriate means for automatically changing a tool as well as for effectively establishing a predetermined positional relationship between the tracer and a cutting tool whenever one cutting tool is exchanged for another cutting tool.